Even my Mechanic is stumped!

I have a 2001 3.5 Olds Aurora. It keeps overheating. Mechanic has changed hoses(even to the heater core), changed the thermostat, checked the water pump, and tested the head gasket. All of that is now in good order, but is still overheating and I believe leaking coolant. The mechanic says that the coolant is not circulating. What else would cause this? He has not checked the radiator as it is a closed radiator. I was hoping to get some suggestions before I pay for that to be done as well.

My money is on the radiator.

It may be a closed system, but I can guarantee it isn’t a closed radiator.

Surely someone has checked the cooling fan/s.

When you say checked the water pump, exactly how was this done? If he only checked for leaks at the pump, and free turning, that might not find a bad pump. what a pump does is pump coolant so that it flows through the system, and you say your coolant is not flowing. Either there is a clog in the system, usually caused by not using anti-feeze in the coolant, or the pump is not moving the coolant. A clogged radiator will decrease the flow, and if the lower hose had a coil in it to keep it from collapsing, and that rusted out it could have damaged the pump and or clogged the radiator. What kind of shape is the fan belt in, and what is the tension on it?